This invention relates to the fields of chemistry and reaction kinetics.
This invention was born of a need for a process to destroy explosives and gun propellants which does not pollute the atmosphere, such as do open burning and open detonation, and which does not simply transform the materials into other compounds whose safe disposal is difficult. The invention is also applicable to other types of hazardous materials.
Oxidation of waste materials with added oxygen in water which is at a temperature and pressure above its critical point (supercritical water oxidation) is known. The oxygen for the reaction is supplied in gaseous form or as a cryogenic liquid or as part of an oxygen-rich liquid such as hydrogen peroxide. The use of water above its critical point (374.degree. C. and 3209 psia) as the medium provides a number of advantages. The solvent properties of supercritical water are different from those of liquid water in that non-polar lipophilic compounds are soluble and salts are less soluble. The majority of hazardous wastes which it is desired to destroy will be soluble in supercritical water. Because complete mixing is possible in the single-phase supercritical region, reaction kinetics are not diffusion limited. In contrast, in two-phase systems, concentration gradients across phase boundaries limit the extent to which compounds can be destroyed.
It is expected that the supercritical oxidation of the present invention will result in conversion of hazardous materials to relatively benign substances and/or to substances which are easily treatable such that they are rendered fit for ultimate disposal into the environment. The products of the inventive process are expected to be primarily CO, CO.sub.2, H.sub.2 O, N.sub.2, N.sub.2 O, H.sub.2, and certain salts. Though oxidation in the medium of supercritical water is similar to combustion, it takes place at lower temperatures than incineration and thus is expected to produce only trace amounts of NO.sub.x, thus solving the significant air pollution problem of NO.sub.x emission. Though the process will produce N.sub.2 O, which is a "greenhouse effect" gas, N.sub.2 O is considered to be a much less serious problem than NO.sub.x. In addition, N.sub.2 O in the atmosphere will not tend to form NO.sub.x but will tend to break down into N.sub.2 and O.sub.2.
The inventive process is contained and thus its effluents can be completely controlled, in contrast to open burning. Because water is the reaction medium, the process can be used for a variety of wastes containing water or for contaminated water. There appear to be no technological upper or lower limit to the concentrations of waste material which can be destroyed. The process can be applied to difficult problems such as cleaning contaminated soil from a polluted site, destruction of armaments, such as solid fuel in a missile, and various types of radioactive waste.